This invention relates generally to gallium arsenide (GaAs) integrated circuits and more particularly to a GaAs voltage reference generator.
The need for stable on-board voltage references in an integrated circuit is well known. Silicon integrated circuits usually exploit the stability of the band gap and are implemented with a junction diode and operational amplifiers. One example of a band gap reference circuit implemented in silicon is illustrated in P. R. Gray, et al., Analysis and Design of Analog Integrated Circuits. Second Edition (1984), page 736. Other examples of IC voltage regulators implemented in silicon are illustrated in R. J. Widlar, "New Developments in IC Voltage Regulators," IEEE Journal of Solid-State Circuits, Vol. SC-6, #1, Feb. 1971.
In a GaAs MESFET integrated circuit, the circuit designer must use Schottky diodes instead of PN junctions, which are unavailable in GaAs MESFET integrated circuit technology. FIG. 1 in the drawings shows a GaAs integrated circuit representative of the prior art, showing reference diodes D1A, D2A, carrying current I.sub.A developed by current-source connected FET Q1A. Inasmuch as current source Q1A is a depletion-mode device having a pinchoff voltage Vp less than 0 volts, current flows even though the gate-to-source voltage Vgs=0 (see FIG. 2).
This circuit has several drawbacks. The output reference voltage, VrefA, is subject to a wide range of variation due to operating temperature shifts and GaAs integrated-circuit-process variations. It is also sensitive to supply voltage variations and noise on the power supply line, -Vs, due to the slope of the Ids vs. Vds characteristic curves, illustrated in FIG. 3. Temperature variations affect operation of both the Schottky diodes and the current source.
Considering the current source, as operating temperature increases, carrier mobility drops and current decreases. This reduces the voltage drop across the diodes and thereby decreases the reference voltage VrefA. Similarly, as the power supply voltage -Vs changes, the current changes: FET Q1A is not a perfect current source. The power supply voltage -Vs is nominally -5 volts but in GaAs integrated circuitry can range from -4 to -6 volts. A 1 volt shift can produce as high as 50 millivolts of change in VrefA.
The diodes are also affected by variations in temperature and fabrication process. For example, as temperature increases, the voltage drop across the diodes decreases for a constant current I.sub.A. This variation, coupled with the temperature sensitivity of a current source Q1a, compounds the sensitivity of VrefA to temperature variations. Such voltage variations have been observed to be as much as +/-250 millivolts.
Gallium-arsenide integrated circuits are much more susceptible to process variations than silicon integrated circuits. GaAs is not a monocrystalline solid, as is silicon, and GaAs fabrication processes have not attained the level of sophistication that silicon processes have attained. The series resistance of the Schottky diodes can vary as much as +/-50%, depending on doping concentrations and the purity of materials used in the GaAs fabrication process.
Experience in the silicon integrated circuit technology is essentially inapplicable to gallium-arsenide technology. Diodes and current source-connected FETs are required in GaAs technology because of the inability to produce a stable high gain amplifier, using a band gap reference, as in silicon, because GaAs FETs are inherently low gain devices.
Accordingly, a need remains for a simple, stable and more accurate GaAs voltage reference generator.